As colorants for printing inks, water-insoluble colorants excellent in fastness such as waterproofness and light fastness, for example, pigments have been used widely to date. To use a water-insoluble colorant as a colorant for a water-based ink, it is required to stably disperse the water-insoluble colorant in an aqueous medium. Water-based inks of the colorant dispersion type are hence used, each of which contains a water-insoluble colorant evenly dispersed in an aqueous medium by the addition of a dispersant such as a high-molecular compound or surfactant.
In recent years, water-based inks of such colorant dispersion type have also finding utility in inkjet recording from the standpoint of image fastness. In inkjet recording, it is attempted to provide colorant dispersion particles, which are dispersed in an ink, with agglomerating property and water-insolubility so that the ink would be able to exhibit improved fixing property and waterproofness on paper. The provision of such properties to the colorant dispersion particles, however, leads to a reduction in the dispersion stability of the colorant dispersion particles in the ink, thereby developing potential problems such that the colorant dispersion particles may agglomerate and settle during storage of the ink, the resulting images tend to exhibit uneven print density, and due to drying of the ink, clogging tend to occur at nozzle tips of an inkjet recording system to result in reduced ejection stability.
In an attempt to solve the above-described problems, an ink—which contains a block copolymer formed of at least one hydrophobic segment and at least one hydrophilic segment and also, an oil-soluble dye—is proposed in JP 2002-97395 A. This ink has stability as a dispersion, but as an ink for use in an inkjet recording system, its ejection stability is still too low to reach a practicable level.
With the foregoing problems in view, the present invention, therefore, has as objects thereof the provision of an ink capable of stably recording images of high fastness and excellent quality over a long period even under severe environmental conditions and also the provision of an inkjet recording method, ink cartridge and inkjet recording system all of which make it possible to record images of superb fastness and quality.